The present invention relates to substrates for making masks and mirrors used in photolithography for making semiconductors. In particular, the present invention relates to substrates that have a very low coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d), which may be used in reflective extreme ultraviolet (xe2x80x9cEUVxe2x80x9d) lithography.
Polished silicon wafers are currently used to make reflective masks and mirrors (a.k.a. reflective lenses) for EUV photolithography. Using silicon ensures that the mask or mirror will have the desired surface quality. Because silicon has a relatively high CTE, however, masks and mirrors made from it may deform, when subjected to heat generated during EUV illumination. This effect may be magnified when the substrate is exposed to higher operating temperatures. For that reason, silicon may not be suitable for making masks and mirrors that will define features which are less than 0.10 micron wide. At those dimensions, even slight warpage of the substrate, which can result from expansion upon heating, can unacceptably distort the image that the mask is suppose to generate.
Certain low CTE materials may be used instead of silicon to form masks and mirrors. It may not be possible, however, to adequately polish the surface of such low CTE materials to generate the high surface quality that EUV lithography requires. The polishing process may scratch the surface of such materials, or introduce other defects, rendering the resulting mask/mirror unsuitable for use in a reflective EUV process.
Accordingly, there is a need for a substrate that may be used to make masks and mirrors, for use in reflective EUV lithography, that has both a low CTE and a high quality surface. There is a need for such a substrate whose surface is substantially free of scratches or other defects, and which does not change dimension when heated. The present invention provides such a substrate and a process for making it.
The present invention covers a substrate that may be used to make masks and mirrors for reflective EUV lithography. That substrate has a first layer that has a low coefficient of thermal expansion. A second layer, which has a high surface quality, is formed on the first layer. The second layer may have a coefficient of thermal expansion that is higher than the coefficient of thermal expansion of the first layer.